caolan/async

Async is a JavaScript utility library that provides helper functions for working with asynchronous code, code where operations happen out of order, like reading files, fetching data from an API, or querying a database, where you have to wait for each operation to finish before using its result. JavaScript's asynchronous nature makes it tricky to coordinate multiple operations, for example, running five tasks in parallel and collecting all their results, or processing a list of items one at a time in order. Async provides ready-made functions for these common patterns so you don't have to write the control-flow logic yourself. The README shows two examples. The first iterates over a set of file paths, reads each file in parallel, and collects the parsed results using async.forEachOf. The second uses async.mapLimit to fetch a list of URLs with a concurrency cap of 5, meaning only 5 requests run at the same time, and collects the response bodies. Both examples work with Node-style callbacks as well as modern async/await syntax. The library works in both Node.js server environments and directly in web browsers. It's installable via npm and also available as a pure ES module package called async-es for use with bundlers like Webpack and Rollup. You would use Async when you need to orchestrate multiple asynchronous operations in JavaScript, running things in series, in parallel, with rate limiting, or with error handling across a batch of tasks, and want a well-tested, compact set of utilities rather than writing that control flow by hand.